Identification of Two novel reassortant avian influenza a (H5N6) viruses in whooper swans in Korea, 2016

On November 20, 2016 two novel strains of H5N6 highly pathogenic avian influenza virus (HPAIVs) were isolated from three whooper swans (Cygnus cygnus) at Gangjin Bay in South Jeolla province, South Korea. Identification of HPAIVs in wild birds is significant as there is a potential risk of transmission of these viruses to poultry and humans. Phylogenetic analysis revealed that Gangjin H5N6 viruses classified into Asian H5 clade 2.3.4.4 lineage and were distinguishable from H5N8 and H5N1 HPAIVs previously isolated in Korea. With the exception of the polymerase acidic (PA) gene, the viruses were most closely related to A/duck/Guangdong/01.01SZSGXJK005-Y/2016 (H5N6) (98.90 ~ 99.74%). The PA genes of the two novel Gangjin H5N6 viruses were most closely related to AIV isolates previously characterized from Korea, A/hooded crane/Korea/1176/2016 (H1N1) (99.16%) and A/environment/Korea/W133/2006 (H7N7) (98.65%). The lack of more recent viruses to A/environment/Korea/W133/2006 (H7N7) indicates the need for analysis of recent wild bird AIVs isolated in Korea because they might provide further clues as to the origin of these novel reassortant H5N6 viruses. Although research on the origins and epidemiology of these infections is ongoing, the most likely route of infection for the whooper swans was through direct or indirect contact with reassortant viruses shed by migratory wild birds in Korea. As H5N6 HPAIVs can potentially be transmitted to poultry and humans, continuous monitoring of AIVs among wild birds will help to mitigate this risk.


Short report
As a natural reservoir for avian influenza viruses (AIVs), wild birds do not typically exhibit clinical signs of infection [1]. In most instances when AIVs of wild-bird origin are transmitted to poultry, infections are initially mild and represent a low pathogenic phenotype. Highly pathogenic AIVs (HPAIVs) arise following adaptation in domestic poultry, and some strains are known to cause significant illness or death if transmitted back to wild birds. Infection by HPAI H5N1 led to the deaths of thousands of wild birds at Lake Qinghai, western China in 2005 [2], and infection with HPAI H5N8 killed over a hundred wild birds of multiple species in the Republic of Korea in 2014/2015 [3].
The viruses contained conservative residues within the receptor-binding pocket of the hemagglutinin protein (HA, including Q226 and G228, H3 numbering), which is associated with a preference for "avian" type cell surface receptors containing alpha 2,3-sialic acid residues [8]. The viruses did not contain amino acid substitutions conferring resistance to adamantane and neuraminidase (NA) inhibitors [9]. The deduced amino acid sequence of the HA protease cleavage site contains a series of basic amino acid residues (LRERRRKR/GLF) characteristic of highly pathogenic AIVs. Experimental infection of 8-week old chickens by intravenous inoculation of the Gangjin 48 virus resulted in the deaths of all 8 birds within 25 h, conferring a intravenous pathogenicity index of 3.0. [5].
Phylogenetic analysis of HA gene sequences from the three viral isolates classified them within Asian H5 clade 2.3.4.4 lineage (Fig. 1a), which emerged in China during 2010-2011, and unlike earlier clades is known for a novel  (Fig. 2). The PA genes for Gangjin 48 and Gangjin 49-2 were closely related (99.16% identity) to A/hooded crane/Korea/1176/2016 (H1N1), while the PA gene of Gangjin 49-1 was more closely related (98.65% identity) to A/environment/Korea/W133/2006 (H7N7) (Fig. 1b). The data does not preclude the possibility that a as yet unidentified, but more contemporary, virus might be the actual donor of the PA gene to Gangjin 49-1 (Fig. 1b).
Whooper swans breed in northern Eurasia and winter in Europe and eastern Asia, including South Korea. Previous satellite data describing their southern migration to South Korea indicate the birds follow the Flyway along inner Mongolia, northeast China and the Korean peninsula during October and November [12]. Indeed, on 27 November many migratory bird species including whooper swans (>1,700), mallards (Anas platyrhynchos, > 3,500) and spot billed ducks (Ana poecilorhyncha, > 1,500) were observed around Gangjin Bay.
It is possible that whooper swans introduced both H5N6 viruses into Korea. However, it is also possible that whooper swans which are highly susceptible to HPAI H5N1 infection [13], may have been exposed locally through direct or indirect contact with other H5N6infected but asymptomatic migratory species, such as mallards or spot-billed ducks, or perhaps through exposure to infected poultry [14]. Wild bird species differ in their response to HPAIV infection, and it is likely that unidentified asymptomatically-infected migratory birds could have introduced an ancestral virus, similar to Guangdong 2016, into Korea. Moreover, these or other species of wild birds could then play a role in generating novel reassortant viruses when co-infected with other AIVs. Indeed, our research group has isolated more than sixty AIVs from wild birds and waterfowl feces since October 2016 (unpublished data). Genetic analysis of these AIVs might provide clues as to the origin of these viruses; the characterization of which will be further detailed in a subsequent publication.
In conclusion, we have identified the two novel HPAIV H5N6 strains from three whooper swans in Korea. Phylogenetic and genetic analyses have shown that the two strains resulted from reassortment of the clade 2.3.4.4 H5N6 viruses that circulated in Guangdong, and they are distinguishable from HPAIVs that caused previous poultry and wild bird outbreaks in Korea. Increased understanding of clade 2.3.4.4 H5N6 virus transmission and pathogenesis in wild birds would serve to increase knowledge of the risk these viruses present to wildlife, domestic animals, and humans. Furthermore, as the H5N6 HPAIVs are potentially transmissible to poultry and humans, continuous monitoring of AIVs among wild birds is warranted.